For practical reasons, electric networks of a photovoltaic system in small or medium size networks often have an ungrounded design. This has the advantage that, for example, in the case of a ground connection of one of the electric poles, merely the electrical potential of this particular pole is shifted to the ground potential. Consequently, the electric network is “single error safe” because a single ground fault has no, or only a minor effect on the operation.
A further advantage of an ungrounded network is the utilization of the full electrical potential difference between both electric poles, resulting in a maximum power output of the photovoltaic system.
In small or medium systems such a design is possible when the systems are provided with an external lightning protection, for example, the building surge protector. In large systems, such lightning protection is more difficult to realize. Therefore, these systems typically have a one end ground.
However, this results in a higher error rate because the network is no longer single error safe. Moreover, it can result in the fact that some of the power to be generated is abandoned. This is due to the fact that the ground potential can be constantly higher or can have a value other than zero.
Another reason for the grounding of the isolated electric network is the high supply voltage of large photovoltaic systems which can reach a range of several thousand volts, wherein currently a range of up to 1,000 volts is common.
In low-voltage power networks, customary earth-leakage monitors are installed between the network and a ground connection point. In such installations, it is of disadvantage that capacitive grounding cannot be measured. In addition, it is possible that an “incorrect validation” of a shifting of the residual voltage between the network connection and the ground potential can result in false alarm. This is caused by fluctuating ground potential and possibly weather-dependent coupling impedance between the network and the ground. Furthermore, “symmetric” isolation faults cannot be discerned. Symmetric isolation faults are faults occurring on both electric poles of the electric network. Finally, with regard to such common earth-leakage monitors, sensitive measurements are not made with passive earth-leakage monitors, but it is required to use active earth-leakage monitors. This is not possible, or only to a limited extent, when using customary AC converters for photovoltaic systems.